This invention relates to a d.c. solenoid, and more particularly to a d.c. solenoid which can reliably operate.
One form of a prior art d.c. solenoid is disclosed in JP-A-59-107108. As shown in FIG. 1, the prior art d.c. solenoid comprises a main coil 1 for attracting a plunger, a thermistor 2 connected in series with the main coil 1 for preventing burn-out of the main coil 1, and a main switch 3 through which the series connection of the main coil 1 and the thermistor 2 is disconnectably connected to a d.c. power source 4.
The thermistor 2 has a positive temperature coefficient as shown in FIG. 2. It will be seen in FIG. 2 that, in a temperature range higher than a predetermined temperature, the resistance value of the thermistor 2 sharply increases with the increase in the temperature. Thus, although a large current is supplied to the main coil 1 when the solenoid starts to be energized at time To in FIG. 3, the current supplied to the main coil 1 sharply decreases with lapse of time as shown in FIG. 3. This is because the temperature of the thermistor 2 sharply rises as a result of self-heating and due to the ambient temperature, and its resistance value shows a great increase.
Therefore, when the diameter, number of turns and other factors of copper wire forming the main coil 1 and the operating characteristic of the thermistor 2 are suitably selected, a large magnetic force can be produced at the solenoid energization starting time, and, after energization of the solenoid, generation of heat from the main coil 1 can be suppressed, thereby preventing undesirable burn-out of the main coil 1 and producing the required plunger holding force.
However, in the prior art d.c. solenoid described above, the temperature of the thermistor 2 heated up to a high temperature level by the current supplied from the d.c. power source 4 would not be immediately lowered even when the main switch 3 is turned off. Thus, when the solenoid is re-energized at time T.sub.1 in FIG. 3, the resistance value of the thermistor 2 remains large even at that time, and a minimum energizing current Ao, shown in FIG. 3, required for re-energization of the solenoid may not be supplied to the main coil 1, resulting in inability to re-energize the solenoid.